Numerous mechanisms known in the prior art enable a body (commonly referred to as a “platform”) to be oriented in three dimensions by being turned about two or three axes. When the body for orienting presents an axis of symmetry, turning the body about its own axis of symmetry is generally not useful (it suffices to think of the reflector of a parabolic antenna or a telescope): under such circumstances, which occur frequently in practice, it suffices to use an orienting mechanism having two degrees of freedom.
A “wrist” is an orienting mechanism presenting motion of spherical type: when such a mechanism is used, each point of the body for orienting moves over a spherical surface. The center of spherical motion may be situated inside or outside the body for orienting.
In an orienting mechanism, it is not always necessary to allow for the possibility of the body for orienting to be able to perform complete revolutions about its movement axes. Under such circumstances, it is advantageous to use a parallel mechanism, i.e. a mechanism comprising a plurality of independent kinematic linkages connecting said body for orienting to the fixed base. Parallel mechanisms are small in terms of size and mass, while simultaneously presenting a high degree of stiffness. These characteristics are particularly appreciated in the aerospace industry, for example.
Very few parallel mechanisms with two degrees of freedom presenting spherical motion (wrist) are known in the prior art. Those mechanisms belong to two classes:                so-called “spherical architecture” wrists that comprise only revolute joints whose axes intersect in the center of spherical motion; and        wrists of non-spherical architecture, that do not satisfy that condition.        
Two examples of spherical architecture wrists are given in U.S. Pat. No. 5,966,991 (a so-called “agile eye” mechanism) and in the article by J. M. Wiitala and M. M. Stanisic entitled “Design of an overconstrained dextrous spherical wrist”, Journal of Mechanical Design, Vol. 122, pp. 374-353, 2000.
Those mechanisms are overconstrained and they obtain spherical motion of the platform by using the common constraints of the spherical mechanisms. The main drawbacks of those architectures are the fact that the spherical motion axes are not predetermined, i.e. they are not fixed, neither relative to the base nor relative to the platform, and the movements in rotation about said axes cannot be decoupled. In addition, the fact that they are intrinsically overconstrained leads to high levels of internal mechanical stress and a major risk of jamming.
Wrists of non-spherical architecture are described in particular in the following articles:
G. R. Dunlop, T. P. Johnes, “Position analysis of a two DOF parallel mechanism—the Canterbury tracker”, Mechanism and Machine Theory, Vol. 34, pp. 599-614, 1999;
R. Baumann, W. Maeder, D. Glauser, and R. Clavel, “The PantoScope: a spherical remote-center-of-motion parallel manipulator for force reflexion”, Proc. IEEE Int. Conf. Robotics and Automation, pp. 718-723, 1997;
M. Carricato, V. Parenti Castelli, “A novel fully-decoupled two degrees-of-freedom parallel wrist”, The International Journal of Robotics Research, Vol. 23, pp. 661-667, 2004;
J. R. Bauer, “Kinematics and dynamics of a double-Gimbaled control moment gyroscope”, Mechanism and Machine Theory, Vol. 37, pp. 1513-1529, 2002; and
G. Gogu, “Fully-isotropic overconstrained parallel wrists with two degrees of freedom”, Proc. IEEE Int. Conf. Robotics and Automation, pp. 4025-4030, 2005.
The first two above-mentioned mechanisms present two axes of motion that are not predetermined, and consequently motions in rotation that are not decoupled. In addition, they are bulky.
The other three mechanisms present predetermined axes and enable motion in rotation to be decoupled, but they are nevertheless penalized by their size. In addition, the mechanism of M. Carricato and V. Parenti Castelli, and the mechanism of G. Gogu contain prismatic joints that can affect their operation negatively, in particular in space applications (lubrication problems) and in micromechanical applications (high levels of friction).